1. Field of the Invention
This invention refers to a method for ascertaining the absolute coordinates of an object, wherein light is directed onto the object through a projection grating and wherein the light reflected from the object is received by a sensor and the signal (shot or picture) of the sensor is evaluated. The invention also refers to an apparatus for performing such a method, i.e. to an apparatus for the determination of the absolute coordinates of an object, consisting of a light source, projection optics, in particular a projection objective (lens), a projection grating which is projected onto the object by means of the projection optics to form an image, a sensor for receiving the light reflected from the object and an objective directing the light reflected from the object onto the sensor to form an image.
2. Description of the Prior Art
By means of the Moire technique and with projected lines the three-dimensional geometry of the surface of an object can be ascertained. This has been described in Takasaki, H.: Moire Topography, Applied Optics, Vol. 9, No. 6, 1970, p. 1457-1472, to which reference is made.
The evaluation of the contour line images is carried out by a computer for instance with the so-called phase shift method described in e.g. Dandliker R., Tahlmann R., Willemin J. F. : Fringe Interpretation by Two-Reference-Beam Holographic Interferography: Reducing Sensitivity to Hologram Misalignment, Oct. Comm. 41.301 (1982), and in B. Breuckmann, "Ein Geratesystem for die rechnergestutzte optische Me.beta.technik" (an apparatus system for the computer-aided optical measuring technique), VDI-Reports 617, Laser Measuring Technique, p. 245-254, to which reference is made. In said phase shift method, images displaced in phase (sensor shots or pictures) are read into the computer successively via a video camera. It is also possible to process the contour line images by means of a Fourier evaluation. With respect to the Fourier evaluation see Th. Kreis, K. Roesener, W. Juptner/D: Holografisch interferometrische Verformungsmessung mit dem Fourier-Transformations-Verfahren (holographic interferometric deformation measurement by using the Fourier transformation method), Laser 87, Optoelektronics in Technology, edited by Springer Verlag, to which reference is made. Furthermore, the evaluation can also be performed using other techniques which only require one contour line image (sensor shot or picture, or video image). Such techniques are described for instance in the German Patent Specification 39 07 430 and in the German Patent Specification 38 43 396 to the contents of which it is also referred.
By the Moire technique the relative form of the surface of the object can be obtained, but in general the absolute distance between the sensor (camera) and the object cannot be ascertained from a contour line image by using said technique. And it is also impossible to determine the absolute coordinates of the surface of the object by the above-mentioned techniques and methods known per se.
However, the absolute coordinates of the object are required for ascertaining the absolute size of the object or the depth of the object when there are step-like cross-sections. Since there is not included any information on the image scale in the contour line image, for the determination of the absolute coordinates additional data are required apart from the contour line image. Said data are gained in known methods by means of distance sensors or by changing the contour line distance or by moving the object or the camera. But for that purpose masses must be moved, which requires a stable structure of the test station and is relatively time-consuming. When a certain degree of precision is to be achieved, the mechanical expenditure is very high. A method and an apparatus for the quantitative absolute measurement of the three-dimensional coordinates of a test object by means of the Moire method is known from the older but not yet published German patent application P 40 11 406.6. With the method described therein a displacement means for displacing and measuring the displacement path of a calibration body and/or of the test object is provided normal to the plane of the gratings (projection grating and reference grating). Reference is made to said patent application P 40 11 406.6.
From the U.S. Pat. Specification 4,802,759, a method for ascertaining the coordinates of a point of an object is known, in which light is directed through the projection grating which is projected onto the object. The pattern created on the object as an image of the protection grating is projected onto a flat and locally resolving sensor for the formation of an image. The coordinates of a point of the object are determined by triangulation of the point from the projection grating and from the sensor. But only one single sensor image is taken. For the performance of the known method is is necessary to identify a reference line.
U.S. Pat. Specification 4,564,295 discloses a method wherein a grating is projected onto the object. Then an image is formed of the object which is covered by a reference grating (Moire). For the evaluation, the reference grating is moved or the projection grating and the reference grating are moved in synchronism. Said synchronous movement of the gratings causes stationary contour lines on the object.
From the U.S. Pat. Specification 4,641,972 a method is known in which a sinusoidal grating is projected onto the object and the object is observed at an angle. The evaluation is performed by means of the phase shift technology. An evaluation by means of a triangulation is not carried out.
U.S. Pat. Specification 4,349,27 discloses a method in which colored gratings with at least two different wave lengths are projected onto the object. The taking of the image is carried out via color separating filters for the wave length selection on two diode arrays. Equidistant gratings in different colors which are displaced with respect to each other are projected in parallel. The evaluation is performed through the ratio of the intensities of the respective colors.